The present invention relates to a liquid jet nozzle for inducing cavitation in a fluid, and a stress improving treatment method using the nozzle.
In apparatus and devices of a nuclear power plant made of austenitic SUS304 stainless steel or Ni base alloy, stress corrosion cracking occurs in some cases. It is well known that such stress corrosion cracking is affected very largely by residual stresses, that is, it is remarkably accelerated in a state of residual tensile stress and is suppressed in a state of residual compressive stress.
Controlling residual stress not only suppresses stress corrosion cracking, but the stress corrosion cracking can be prevented by changing the residual tensile stress to residual compressive stress.
Further, the residual stress improvement serves not only for stress corrosion cracking resistance but for prevention of damage due to corrosion, fatigue, corrosion fatigue, etc.
Therefore, in a nuclear power plant, it is important to improve the stress by changing the residual tensile stress caused in welded portions of an apparatus and various devices to residual compressive stress.
A metal surface strengthening method is disclosed in JP A 59-193215, wherein residual tensile stress caused by working, such as welding, is reduced by causing residual compressive stress to occur on a surface of a metal material, thereby to prevent damage, such as stress corrosion, corrosion, fatigue, corrosion fatigue, etc.
This method is designed to strengthen metal by peening, that is, by causing cavities to be produced in liquid by jetting liquid at high pressure and high velocity from a nozzle onto the metal submerged in the liquid and utilizing the collapse impact force of the cavities to apply a force to the metal.
A nozzle for jetting liquid at high pressure and high speed is disclosed in JP A 59-25681.
JP A 59-25681 discloses a submerged jet nozzle which is provided with an orifice having a section in the shape of a circle, an ellipse or a rectangle, and which has a diffusion chamber disposed downstream of the orifice and expanding at an angle of 20-60.degree. with respect to the axis of the orifice, in order to increase the submerged jet flow.
JP A 59-25681 also discloses that the length of a jet hole extending from the orifice is 4 to 20 times the diameter of the orifice, and this construction reduces the flow velocity loss of the submerged jet flow and can cause the jet flow to reach a more remote area.
However, JP A 59-193215 does not disclose any specific means for jetting liquid of high pressure and high speed.
On the other hand, JP A 59-25681 has an object to suppress decay of the jet flow, and does not disclose any means for obtaining jet flow with cavitation.
Thus, in the prior devices, in order to achieve a sufficient effect, it is essential to shorten the distance (jet distance) between the nozzle and an object to be treated, and it is difficult to attain a peening effect in a case where the jet distance is long and in a case where decay in the jet flow velocity is remarkably large, as in a submerged jet, even if the jet distance is short.
In jetting in the atmosphere, the jet velocity decay is small, however, it is required to raise the jet pressure to a super high pressure in JP A 59-25681 in which only an impingement dynamic pressure is used. Therefore, the apparatus is disadvantageous also in performance and in the cost of a pump, and relevant apparatuses.